Tool Holders for Dickson Clone

[MWParticipant @mw27036]

I've checked this and it only rotates 90 degrees, but thats what it takes to holder, if i remove the holder it will rotate almost 360, so there isn't anything wrong with it.

Michael W

[Martin ConnellyParticipant @martinconnelly55370]

I think the cam needs to rotate to near the top dead centre to give the best chance of locking the holder and for the best mechanical advantage in the pull up. If your cam is a long way off that best position then you do not have the best possible grip on the tool holder. The pull is off to one side and the forces acting on the cam will be able to rotate it to the release position.

Martin

[Clive FosterParticipant @clivefoster55965]

Just looked at my Rapid and Dickson T2 size posts. Both go 90° (by visual estimate) between tool release and lock. Interestingly the Rapid only goes about 20° past lock if the mechanism is turned without a tool holder in place whilst the Dickson goes about 35° further. Travel is limited by the flange that goes in the tool holder Tee slot hitting the toolpost body.

I've never got my head around how these "rotate to jam" cam locking systems actually manage to generate locking forces. Clearly with solid to solid contact there can be no overcentre effect as with sprung systems. Probably some sort of effective contact angle involved whise sine has a similar relationship to the coefficient of friction between the cotacting materials in the sane manner as for the taper angle of self holding. If this is the case one would expect locking to occur once the angle of turn gets close enough to 90°. Which seems to be the case.

Really must find the time to do a proper analysis of the Dickson system to put numbers on the effects of dimensional tolerances.

Clive

Edited By Clive Foster on 16/07/2016 22:26:33

[Martin ConnellyParticipant @martinconnelly55370]

Clive, if you do the calculations of forces on a morse taper you find that when the tan of the angle (I can't remember the exact details) is less than the coefficient of friction the taper is classed as self holding. However the self holding force is easily overcome by additional force such as used to eject the tapered item from the socket. I think there are similar forces involved with these tool holders and they are relying on friction. Given sufficient force this friction will be overcome as well. That is why I made a new plunger to take the holder closer to a mechanical lock. True mechanical lock would require the cam to be at tdc but would also mean perfectly sized parts all round. That is why I aimed for close to tdc. At this point the pressure point on the plunger will be close to the centre line and the force required from the plunger to turn the cam will be very high. With only 90 degrees of motion to the pull up point the contact point between cam and plunger will be off the centre axis and the force required to overcome friction and turn the cam much lower. If I remember correctly it only took a couple of hours to drill and turn the new plunger so I think it was a small investment of time to get a better tool holder clamping action.

I think the off centre pressure point means that the plunger may twist anticlockwise when viewed from above. This would put all the pull up force on the tool holder on one lip and that is the one furthest from the workpiece. This is the worst possible result mechanically which is why the idea of the pressure point close to the centre line of the plunger seems much better to me.

Martin

[TobyParticipant @toby]

A heads up for anyone tempted to buy dickson toolholders cheap on ebay from india.

I just bought a couple for my boxford from "globaltools2016" because UK suppliers appeared to be out of stock. They don't have a chance of fitting without significant modification.

My post is an original Boxford badged one and (from the look of them) I have a mix of original holders plus later clones which all fit fine so I am pretty sure it isn't the post. However the globaltools2016 ones need a significant amount removing from the Vs to fit.

I also agree with the comments about the difficulty of making them accurately, having milled the Vs deeper I am going to have to resort to hand finishing to get them to fit without rocking.

[Harry WilkesParticipant @harrywilkes58467]

When I purchased my S7 lathe it came with a Dickson QCTP but only a couple of tool holder so I purchased a couple from the Myford stand at the Midland Model Exb 2 years back although quite well made they did not fit correctly so a friend eased them on his surface grinder. I do not know if the problem still persists or if it was just a 'batch' problem I have not purchased any more fortunately I was able to do a trade off for four originals

H

[Michael GryznaParticipant @michaelgryzna78305]

Toby, I just had the same experience buying a couple of Indian made toolholders for my Boxford badged toolpost. The claw on the back of the holders, which are supposed to engage with the piston head of the toolpost were too thick, compared with a genuine Dickson, by between 0.74 and 0.65mm, (both were different), The Ebay seller, techtoolsuk told me that the holders were manually ground, and compared against a master copy of a Dickson toolpost. I suspect that the master copy was of the 'Dickson style' toolpost imported by the same company. Since then I've been unable to find an importer who will guarantee compatibility with an English made toolpost. I'll just have to keep looking for secondhand and hope I don't get trampled in the rush.

[Nick HulmeParticipant @nickhulme30114]

I'm using two clones from different suppliers on my Super 7, the RDG at the front and a Chronos at the rear, this is a test with a 30mm 314 SS bar to see if the front setup is too flexible as all the "Experts" will tell you it should be –

https://youtu.be/pUI2GIGTz_c

 

Edited By Nick Hulme on 19/08/2017 09:01:35

[andrew briggsParticipant @andrewbriggs24687]

Posted by Toby on 04/04/2017 09:03:54:

A heads up for anyone tempted to buy dickson toolholders cheap on ebay from india.

I just bought a couple for my boxford from "globaltools2016" because UK suppliers appeared to be out of stock. They don't have a chance of fitting without significant modification.

My post is an original Boxford badged one and (from the look of them) I have a mix of original holders plus later clones which all fit fine so I am pretty sure it isn't the post. However the globaltools2016 ones need a significant amount removing from the Vs to fit.

I also agree with the comments about the difficulty of making them accurately, having milled the Vs deeper I am going to have to resort to hand finishing to get them to fit without rocking.

Hi Toby,

I bought an HV6 rotary table from global tools2016 and it was complete junk.

The eccentric bush to disengage the drive was 0.2mm smaller than the machined hole it went in.

looks like a firm to avoid

Andy

[John HaineParticipant @johnhaine32865]

I got a few genuine toolholders literally "off the back of a lorry" at a steam fair last year, scattered through a box of junk. I gathered together all I could find (5 or so IIRC) and asked the guy how much – he said a fiver – I said "the lot?" – he said yes. I paid and ran before he changed his mind!

[Nick Taylor 2Participant @nicktaylor2]

I've bought 3 from RDG in T1 size last month for the genuine Colchester 3 sided post on my Chipmaster and they all fit perfectly on all 3 positions, could be luck I suppose but not bad for £22.50! The screws aren't as soft as usual either!

[Michael GryznaParticipant @michaelgryzna78305]

After my bad experience with a couple of Indian made toolholders, (Got my money back). I bit the bullet and ordered two from Rotagrip. Made in Poland. The finish is streets ahead of the Indian clones, and they fit my Boxford toolpost, smooth as silk. Twice the price of the Indians, but you get what you pay for. Hats off to Ian (Rotagrip/Fordeight) who even managed to get them to me from inside France, saving a few £'s on the postage. Highly recommended.

[thaiguzziParticipant @thaiguzzi]

Just finished another 3 toolholders in alloy for my Bison clone for my Boxford.

Have 3 original, hardened, ground Bisons.

Have 11 home made, non hardened, non ground steel holders. Work a treat, couple of years of abuse, zero problems.

Have another 5 in alloy, ditto above.

Makes 22 in total.

The last 3, I made a cock up on the machining of the small outer vees, going a few thou too deep, daylight on one side, and a couple of cigarette papers clearance on the other when clamped to the toolpost. ie they are located and clamped only on the two large inner vees. Well, finished them off, and they repeat and they turn and face and bore and do NOT move. Put all my low power, small DOC boring tools in them.

Waste not want not etc…

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